NIAAA has identified non-college bound young adults as a key population for targeted intervention to reduce heavy alcohol use and related negative consequences. Brief interventions, based in the principles of Motivational Interviewing (MI), have demonstrated efficacy among college students, but very little is known about MI efficacy, active treatment ingredients, and mechanisms of behavior change with heavy drinking young adults categorized as non-college. The current study will test a series of theory-driven hypotheses regarding the within-session predictors of outcome in an efficacious MI with this understudied population. To do so, this study will collect observationally-rated process data (Motivational Interviewing Skill Code 2.5; Client Language Assessment - Proximal/Distal) from audio-recorded session files derived from a recently completed clinical trial (AA016000; PI: Colby). The clinical trial compared a single session MI to a time-matched control of meditation and relaxation training (REL) in a sample (N=167) of underage, past-month heavy drinkers (17-20 years old). Compared to REL, MI significantly reduced alcohol use and alcohol-related negative consequences at 3-month follow-up while additionally impacting a series of theory-driven cognitive and behavioral intermediate outcomes (motivation to change drinking; perceived drinking norms; self-efficacy to change drinking; behavioral strategies to limit drinking). These results present a unique opportunity to investigate the within-session processes responsible for therapeutic improvement with an understudied, yet high need young adult population. Specifically, this research will incorporate novel conceptual and statistical considerations to examine: the Technical Hypothesis of MI efficacy, the client language mechanisms of the motivation building and change planning elements of MI process (MI Evoking, Focusing, and Planning), the specific within-session MI ingredients (exploring the Pros and Cons of Alcohol Use, providing Personalized Feedback on Alcohol Use, and planning for Behavioral Strategies to Limit Drinking) that predict client language mechanisms and the specific conditions (therapist MI Empathy and Spirit, client baseline motivation to change) under which causal models are optimally specified. Moreover, the presence of the time-matched control condition allows for between-condition tests of within-session ingredients and mechanisms, which is a level of contrast rarely achieved in the MI process literature to date. Information derived from this study can be immediately communicated to service settings in the form of MI dissemination guidelines or directly to providers in the form of empirically-supported principles of MI treatment delivery. This research can also inform the MI literature by examining the consistency of its key causal model across populations while additionally extending the model to identify further links in the causal chain. Finally, the success of MI as a behavior change intervention may be due to its capacity to harness core principles of client-centered therapy, which suggest this line of research may have implications for treatment-facilitated behavioral research more broadly.